1. Field of Invention
The present invention relates generally to equipment used in semiconductor processing. More particularly, the present invention relates to an internally cooled mirror that is suitable for use in an extreme ultraviolet lithography system.
2. Description of the Related Art
Extreme ultraviolet (EUV) lithography is a semiconductor fabrication technology which enables semiconductors with small features, e.g., features with dimensions of approximately 45 nanometers (nm) or less, to be produced. In EUV lithography, a laser heats xenon gas to create a plasma, although there are other method used to make EUV plasmas. Electrons come off the plasma and radiate light. FIG. 1 is a block diagram representation of a EUV lithography system. A EUV lithography system includes a source 104 which provides electrons that radiate light. Source 104 includes a plasma source 108 and a collector mirror 112 that reflects electrons which come off of the plasma generated by plasma source 108 into an illuminator unit 116. Illuminator unit 116 is a condenser that effectively collects light and directs or otherwise focuses the light onto a reticle 120. That is, illuminator unit 116 conditions light from plasma source 108 to improve uniformity. The light reflects off of reticle 120, through projection optics 124, and onto a surface of a wafer 128.
Mirrors included in a EUV lithography system generally absorb some of light or radiation that comes into contact with the mirrors. Such mirrors may be included in illuminator unit 116 as well as in projection optics 124. Often, if the mirrors are multi-coated, the mirrors reflect approximately seventy percent of the incident light on the mirrors while absorbing approximately thirty percent of the incident light. When light is absorbed by a mirror, the absorbed light is converted to heat. Heat generally causes distortion in the mirror, thereby degrading the optical performance of the mirror. When there is distortion in the mirror, the optical performance of the mirror is compromised.
Although radiant cooling methods may be effective in cooling mirrors while causing an insignificant amount of distortion, when the heat load on a mirror is relatively high, radiant cooling methods are often inadequate for cooling mirrors. Internal or direct cooling methods, e.g., liquid cooling methods, may be applied to mirrors to provide cooling in the presence of relatively high heat loads. Conventional internal cooling methods are typically associated with turbulent flow, as turbulent flow provides for relatively efficient heat transfer and cooling. However, the use of turbulent or non-laminar flow to cool a mirror generally causes the mirror to vibrate. When a mirror that is cooled by internal cooling methods which utilize turbulent flow is a part of a EUV lithography system, the vibrations caused by the turbulent flow may adversely affect a lithography process, particularly if the vibrations cause an illuminator unit, projection optics, a reticle, or a wafer to vibrate. By way of example, if a projection optics system or a reticle is subjected to vibrations, a pattern reflected off of the reticle through the projection optics may be inaccurately projected onto the surface of a wafer. As a result, the accuracy of the EUV lithography process may be compromised.
Additionally, if a temperature rise in a mirror is relatively large, heat may be radiated to nearby structures in a EUV lithography apparatus. The radiating of heat to structures in a EUV lithography apparatus may cause adverse structural changes and, as a result, adversely affect the performance of the structures.
Therefore, what is needed is a method and an apparatus which allows a mirror in a EUV lithography system to be efficiently cooled. That is, what is desired is an internal cooling method for a mirror that is a part of a EUV lithography apparatus that allows the mirror to be cooled in the presence of relatively high heat loads, while reducing distortion of the mirror and vibrations within the EUV lithography apparatus.